The use of security threads for protecting banknotes, credit cards and other valuable documents is well known. A security thread is a strip of material placed on the surface of a banknote document or sheet such as banknote; alternatively a security thread may be serpentined or woven into the banknote paper (a window type effect) to confer additional security (authenticity) to the bank note. Typical dimensions of a hot stamp thread are a width of 1-5 mm, a thickness of 3-4 μm; windowed polyester terephthalate (PET) based threads have a thickness of about 0.5 mil or 12.5 microns. By way of example, one of the earliest forms of security threads consisted of reflective foil transferred by hot stamping to the surface the banknote (GB 2119312 A). This reflective foil prevented reproduction of counterfeit banknotes by printing processes such as from printing presses, PC printers and copiers. Holograms (EP-A-0624688), holographic features along with thermo chromic features (GB 2347646), opaque coatings having characters and patterns readable by transmitted light in combination with luminescent substances (U.S. Pat. No. 6,474,695), repeating patterns of magnetic/magnetic indicia or metal dots (W02103624), laser etching fine lines and text with a laser (German “Auslegeschrift” no. 22 05 428) and (WO02101147), printing micro-characters on a metalized transparent plastic with clear acid resistant inks followed by acid etching of the unprinted areas to produce shiny micro-characters on a transparent base (U.S. Pat. No. 4,652,015), bonded nucleic acid molecules so that complementary nucleic acid molecules can bind to the molecules already attached to the document (DE 10122836), and optically variable security elements using liquid crystal material (EP0435029) have all been used to make security threads. However, these aforementioned threads either take too much time to make and or have other associated problems; for example, it is found that laser etching takes too long to be cost effective, etching by use of chemicals requires multiple steps and is not considered to be environmentally-friendly; holograms can be readily copied, and in many instances the features of the threads are not readily seen by eye by the average person and machines are required to read them.
A method to pattern a single layer of metal or carbon in a vacuum chamber was advanced in U.S. Pat. No. 4,022,928 by Piwcyzk. Piwcyzk used various methods to apply a perfluoropolyether known as FOMBLIN™ or Krytox™ to a substrate requiring a pattern for a vacuum deposited layer. The perfluoropolyether inhibited the deposition of the depositing material to a web or plastic substrate. Application of this fluid was by spray or vacuum evaporation in combination with a selected removal process as with a laser or an electron beam. A printing method was also described. Printing techniques including relief printing such as letterpress or flexography, planographic printing such as offset lithography, and gravure, and screen-printing such as silkscreen process printing were disclosed.
Subsequently, Ronchi in U.S. Pat. No. 4,749,591 incorporated herein by reference, and in PCT application WO 8700208(A1)) advanced this printing process by applying the inhibiting oil, FOMBLIN, to a vacuum roll coater where patterning thin films on plastic substrates was desired.
Ronchi in U.S. Pat. No. 4,749,591 only discloses applying a single layer of metal, for example, aluminum as is shown in FIG. 1, deposited as a vacuum thin film layer. A demetallized aluminum layer in the case of a security thread embedded into a banknote can easily be forged by simply using a metallized polyester that is subsequently patterned by one of the above methods. In an attempt to copy a security thread having a single layer manufactured by Ronchi's technique, patterning by photolithography in combination with a caustic etchant, or by any of the aforementioned processes or even by using a silver pencil to simulate the security thread could be used. Security threads having multi-layer films where one of more of the layers are patterned has not previously been considered. A major impediment to providing several thin film layers, was residual oil remaining on the images and on non-patterned areas of the web. This residual oil was detrimental to further thin film coating since left over oil would cause “ghosting”; a process whereby the inhibiting oil is transferred to the back side of plastic film when roll coating, which in turn causes inhibiting oil to be transferred further down the web on the front side. Left over inhibiting oil also causes adhesion failures to subsequent thin film layers.
“Ghosting” and the ability to remove residual inhibiting oil is overcome by this invention. By way of this advance, for the first time, patterned multilayer optical stacks could be conveniently manufactured in a cost effective way on a security thread, by roll coating. In particular, a new optically variable security thread that had a high pattern resolution was made that contained readable text or graphic images where covert features such a magnetic signatures could also be incorporated.
It is an object of this invention, to provide a security thread having optically variable features such as an optically variable pattern that can be seen against a background that is distinguishable from the pattern, or from which the pattern stands out.
It is a further object of this invention, to provide a relatively simple, inexpensive method of manufacture of a multilayer patterned security thread for use within or upon a web or sheet, for example on currency, documents or packaging for providing authentication thereof.